On coal tar hydrogenation

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Coal tar hydrogenation technology involves using fixed-bed hydrogenation to remove impurity atoms such as S and N from coal tar, as well as to saturate the olefins and aromatic compounds present in it, thereby producing high-quality naphtha and diesel fractions. Generally, the S and N contents in the naphtha produced by the hydrogenation of coal tar are below 50 ppm, while the aromatic potential content is above 80% ； The diesel fractions produced have a S content of less than 50 ppm, an N content of less than 500 ppm, a cetane number of above 35, and a freezing point of below –35°C to –50°C; they are high-quality components for blending into clean diesel. 1. Overview of coal tar hydrogenation technology 1.1 Main chemical reactions in coal tar Coal tar hydrogenation is a multiphase catalytic reaction; during this process, the main chemical reactions that occur include hydrodesulfurization, hydrodenitration, hydrometallurgy, hydrogenation saturation of olefins and aromatics, as well as hydrocracking: ① Hydrodesulfurization reaction ② Hydrodenitration reaction ③ Aromatic hydrogenation reaction ④ Olefin hydrogenation reaction ⑤ Hydrocracking reaction ⑥ Hydrometallurgy reaction 1.2 Factors affecting the operating cycle and product quality of coal tar hydrogenation units The main factors that influence the operating cycle, product yield, and quality of coal tar hydrogenation units are reaction pressure, reaction temperature, space velocity, hydrogen-to-oil volume ratio, and the properties of the feed oil. 1.2.1 Reaction pressure Increasing the reactor pressure and/or the purity of the recycled hydrogen also increases the partial pressure of the reaction hydrogen. Increasing the hydrogen partial pressure not only facilitates the removal of impurity atoms such as S and N from coal tar, as well as the hydrogenation saturation of aromatic compounds, thereby improving the quality of the resulting products, but it also slows down the coking rate of the catalyst, extends its service life, and reduces the cost associated with it. However, an increase in the hydrogen partial pressure also raises the capital investment and operating costs for the facility. 1.2.2 Reaction temperature: Increasing the reaction temperature accelerates both the hydrogenation rate and the hydrocracking rate. Excessively high reaction temperatures reduce the degree of hydrogenation saturation of aromatic compounds, cause condensed ring compounds to coking, and shorten the catalyst’s service life. 1.2.3 Volume Space Velocity: Increasing the reaction volume space velocity enhances the processing capacity of the coal tar hydrogenation unit. For newly designed units, a high volume space velocity can reduce the investment cost of the unit as well as the cost of purchasing catalysts. A lower reaction volume space velocity allows the desired product yield to be achieved at a lower reaction temperature, while also extending the catalyst’s service life; however, an excessively low volume space velocity will directly affect the economic viability of the plant. 1.2.4 Hydrogen-to-oil volume ratio: The value of the hydrogen-to-oil volume ratio is primarily determined by the chemical hydrogen consumption of the hydrogenation feed, and it indicates the relative amount of hydrogen required for that feed. Compared to ordinary petroleum-based feedstocks, coal tar hydrogenation requires a higher hydrogen-to-oil ratio. The reason is that coal tar consists mainly of aromatics, and more hydrogen is required during the reaction process ； Furthermore, the hydrogenation saturation of aromatic compounds is a highly exothermic reaction process, and sufficient amounts of hydrogen are required to remove the heat of reaction from the reactor, thereby preventing the hydrogenation unit from experiencing a temperature spike. 1.2.5 Properties of coal tar: The properties of coal tar affect the operation of hydrogenation units. Nitrogen content: Nitrides are primarily located on the aromatic rings, and their removal involves first hydrogenating and saturating these rings, followed by the breaking of C-N chemical bonds. Therefore, an increase in the nitrogen content of the feedstock requires higher activity from the hydrogenation catalyst. Meanwhile, the NH3 produced as a byproduct of the reaction reduces the hydrogen partial pressure, which in turn affects the catalyst’s service life and its ability to achieve hydrogenation saturation. Sulfur content: The sulfur present in the feedstock is converted into H2S during the hydrogenation process. Therefore, the sulfur content primarily affects the hydrogen partial pressure; a higher sulfur content leads to a significant decrease in this pressure, which in turn impacts the catalyst’s service life and its ability to achieve hydrogen saturation. Asphaltene: The main impact of asphaltene on hydrogenation units is to cause catalyst coking and carbon deposition, leading to catalyst deactivation, accelerating the temperature rise in the reactor, and shortening the catalyst’s service life. Trace metal impurities: The trace metal impurities present in the raw materials include Fe, Cu, V, Pb, Na, Ca, Ni, Zn, etc. During the hydrogenation process, these metals deposit on the catalyst, blocking its pores and causing permanent deactivation of the catalyst. 2. Hydrogenation results of coal tar (examples) 2.1 Hydrogenation of the raffinate from high-temperature coal tar Properties of the product after hydrogenation Density (20°C) /g•cm-3: 0.8730 Total naphthenes: 80.6 Boiling range/°C: Ring structure: Monocyclic – 38.2; Dicyclic – 40.4; Tricyclic – 2.0 IBP/10%: 120/196; 30%/50%: 213/218; 70%/90%: 224/232 Total aromatics: 19.4; 95%/EBP: 242/274; Monocyclic: 18.1 Octane number: 33.1 Dicyclic: 0.3 Since the raffinate falls within the diesel boiling range, only one product is obtained after its hydrogenation, and that is a good component for diesel blending. The properties of the diesel fraction produced by hydrogenation show that its density is 0.8730 and its cetane number has increased to about 33, making it an excellent component for diesel blending. 2.2 Hydrogenation of anthracene oil in high-temperature coal tar – Product distribution of anthracene oil hydrogenation: Project, Data, %; 177°C (diesel fraction): 70.74; C5+ liquid yield: 97.69. Properties of the naphtha obtained from anthracene oil hydrogenation (65–177°C): Density (20°C)/g•cm-3: 0.786; Cycloalkanes: 90.0; Octane number (RON): 65; C5/C6 ratio: 0.2/19.0; S/mg×g-1